A balanced steel shutter having a hoisting drum provided with a balancing device with a built-in coil spring has come into wide use as a shutter, particularly as a light-weight shutter, for opening/shutting an entrance, a window, etc. of a building. A balancing device (spread type balancing device) used in such a shutter is configured such that spring torque is accumulated corresponding to the quantity of rotations of a hoisting drum by twisting a coil spring included in the hoisting drum when a slat curtain is fed-out from the hoisting drum to thereby balance this spring torque with weight torque (hoisting drum torque) of the slat curtain fed-out as balance torque of the hoisting drum so as to reduce an operation force required for opening the shutter. Consequently, even a shutter with a considerably large opening can be operated manually.
JP-A-5-163880 discloses a technique in which spring torque of a balancing device is corrected so that the balance torque transmitted to a hoisting drum when the shutter is shut fully is made to be substantially zero. This balancing device is mainly used for a shutter with an air-slit type aluminum roll-formed slat (an air-slit type shutter). The balancing device has a balance pulley rotating synchronously with the hoisting drum, a tension wire one end of which is attached to the balance pulley, and a spring means for giving a spring force to this tension wire. The balance pulley has a main pulley portion for taking-up the tension wire when the slat curtain is shut, and a zero-point return pulley portion provided in one end of this main pulley portion continued to the main pulley portion to take up the tension wire. This zero-point return pulley portion has an outer circumferential shape by which the tension wire passes the axial center of the main pulley portion when the slat curtain is taken out to the position where the slat curtain is fully shut. Because of an outer circumferential shape of the zero-point return pulley portion, the line of action of the tension wire passes the axial center of the main pulley portion when the tension wire is wound around the zero-point return pulley portion. Therefore, though spring torque of the spring means acts on the tension wire, the force is not transmitted to the hoisting drum as torque, so that balance torque is not generated. Consequently, only the own weight of the slat curtain acts when the slat curtain is in its fully shut position with such an advantage that it is possible to shut the air slit in the fully shut position.